Sheet film pack

ABSTRACT

A sheet film pack has a pack housing containing a plurality of sheet film units in a stack therein, a lid slidable relative to the pack housing to open or close an open top of the pack housing, and a plate spring mounted on an inner bottom surface of the pack housing to urge the stack of sheet film units toward the open top. Each sheet film unit consists of a sheet of photographic film and a plastic film sheath backing and fringing the sheet film. The plate spring has a pair of arched arms which extend parallel to each other along side zones of a bottom one of the sheet film units, such that peak areas of the arms contact the bottom sheet film unit at positions within 15 mm from side edges of the sheet film unit.

This is a Divisional of application Ser. No. 08/679,711 filed Jul. 12,1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet film pack containing therein aplurality of sheet films stacked one atop another, and more particularlyto a sheet film pack which is improved in creep resistance.

2. The Known Prior Art

A sheet film 4×5 inches or more in size is used in large format cameras,such as view cameras and studio cameras, for taking a photograph that isto be printed in a large size or at a large print magnification, andthus requires sharpness of the image.

In order to take photographs on a large number of sheet films in rapidsuccession through a single camera, a sheet film pack and a film packholder are suggested in Japanese Laid-open patent application Ser. Nos.5-341378 and 5-341379.

As shown in FIGS. 20 and 21, the sheet film pack 1 contains a pluralityof, e.g., eight sheet film units 5 stacked one atop another in a packhousing 2. The sheet film unit 5 consists of a 4×5 inches sheet film 3and a film sheath 4, as shown in FIG. 22. The film sheath 4 may be madeof a plastic resin as an integral body having a backing plate 4a forbacking the sheet film 3 and a fringe 4b holding three side margins ofthe sheet film 3. The sheet film 3 can be slid into the film sheath 4along a channel formed between the backing plate 4a and the fringe 4b.

As shown in FIGS. 21 and 23, a plate spring 7 is mounted on an innerbottom surface 2a of the pack housing 2 to urge the stack of sheet filmunits 5 toward an open top 2b of the pack housing 2. A pair of guiderails 2c are formed along longitudinal side margins of the open top 2b,for guiding a sliding lid 9 to open and close the open top 2b of thepack housing 2. A grip 9a is formed on a leading end of the sliding lid9, to facilitate the reciprocating operation of the sliding lid 9. A cap11 is removably attached to a trailing end 2d of the pack housing 2. Thecap 11 has a fastening member 12 and an L-shaped plate 13 for enclosingthe fastening member 12 in the cap 11. The fastening member 12 is tofasten the cap 11 to the pack housing 2 to shield the interior of thepack housing 2 in a light-tight fashion, while locking the sliding lid 9in the closed position through engagement in holes 9b and 9c of the lid9.

The pack holder 10 is constituted of a holder housing or external frame16, a sliding tray or internal frame 15 slidable in the holder housing16, and a cover 14 hinged to the holder housing 16. The sheet film pack1 is loaded in the sliding tray 15 with its top side oriented toward thecover 14, that is, toward a front of the pack holder 10 with respect tothe camera to which the pack holder 10 is attached.

When the cover 14 is closed after the sheet film pack 1 is loaded in thepack holder 10, the fastening member 12 is resiliently depressed by thecover 14, thereby unlocking the sliding lid 9. When the sliding lid 9 isdrawn out to a predetermined amount, a topmost one 5a of the stack ofsheet film units 5 pops out through the open top 2b to an exposureposition of the holder housing 16, because of the force of the platespring 7. The exposure position is defined by an inner wall 14a of thecover 14. Thereafter, the sliding lid 9 is moved back to the closedposition, thrusting into between the topmost sheet film unit 5a and thenext sheet film unit 5b. Thus, the sheet film unit 5a is maintained flatin the exposure position. Four leaf spring 17 are mounted to the innerwall 14a of the cover 14 to urge the topmost sheet film 5a toward abottom or back wall 10a of the pack holder 10.

Upon drawing out the sliding lid 9 again, after making an exposure onthe sheet film 3 of the sheet film unit 5a, the sliding tray 15 is drawnfrom the holder housing 16 together with the sliding lid 9 and the packhousing 2, while the cap 11 and thus the exposed sheet film unit 5a, isleft in the holder housing 16. Then, the leaf springs 17 push theexposed sheet film unit 5 down to the bottom wall 10a of the pack holder10.

A partition wall 2e is disposed inside the trailing end 2d, for aligningthe ends of the sheet film units 5 in the pack housing 2. And a recoverymouth 8 is provided between the partition wall 2e and the inner bottomsurface 2a of the pack housing 2, for receiving the exposed sheet filmunit 5a back into the pack housing 2. Specifically, as the pack housing2 and the sliding tray 15 are moved back to the holder housing 16 bypushing the sliding lid 9 into the holder housing 16, the exposed sheetfilm unit 5a that is left in the holder housing 16 is inserted into thebottom of the stack of sheet film units 5 through the recovery mouth 8,by virtue of the leaf springs 17.

In this way, the sheet film units 5 in the pack housing 2 are placed inthe exposure position, in turn responsive to the sequentialreciprocating operation of the sliding lid 9.

The sheet film pack 1 further has a counter disc 21 for displaying thenumber of exposed sheet film units 5. The counter disc 21 is rotatablydisposed in a cavity 20 formed in the pack housing 2 proximate to theleading end thereof, and is rotated by a not-shown counter advancingmechanism mounted in the pack holder 10, one step at each reciprocationof the sliding tray 15. A pressing member 22 is mounted in the cavity 20with its stem portion guided along a vertical rail 20a of the cavity 20.An upper end 22a of an U-shaped spring arm of the pressing member 22 isalways constrained by the under surface of the sliding lid 9, so that alower end 22b of the stem portion of the pressing member 22 alwayscontacts one of a plurality of claws 21a formed circumferentially on thecounter disc 21, so as to prevent reversal of the disc rotation.

The claws 21a are associated with film number indices, and all but oneare equal in height. The one that is higher than the other claws 21a andhas a flat top surface, is associated with an end mark. Thereby, anupper end 22c of the stem portion of the pressing member 22 ismaintained spaced a predetermined distance from the under surface of thesliding lid 9 during photographing. But when the final sheet film unit 5has been exposed, the pressing member 22 moves upward to bring the upperend 22c into engagement with a hole 9f of the sliding lid 9, so as tosecure the lid 9 to the closed position.

FIG. 24 shows the plate spring 7 used in the sheet film pack 1. Theplate spring 7 has a pair of parallel arms 7a and 7b connected at theirone ends through a base portion 7c. The arms 7a and 7b are archedrelative to the bottom surface 2a of the film pack 2, as shown in FIGS.21 and 23, so that the arms 7a and 7b support the sheet film unit 5 atpeaks 19a and 19b of the arches. However, as shown in section by theFIG. 25, under the spring force of the arms 7a and 7b urging the stackof sheet film units 5 upward, the sheaths 4 of the sheet film units 5are deflected because its stiffness is low in the middle portion of thebacking plate 4a, compared with the side zones. The deflection canresult in creep strain whose degree increases with the time. Therefore,if the sheet film units 5 are contained in the sheet film pack 2 for along time, the sheet film units 5 won't be maintained flat, so that whenthe lid 9 is moved back between the topmost and next sheet film units 5aand 5b the sliding lid 9 can scratch the sheet film 3 of the next sheetfilm unit 5b.

The pressing member 22 for the counter disc 21 can suffer creep strainbecause it is kept depressed by the sliding lid 9 until all the sheetfilm units 5 contained in the sheet film pack 2 have been exposed. As aresult of creep strain, the pressing member 22 would not sufficientlypress the counter disc 21, so that the counter advancing operation couldnot be properly effected. The creep strain can occur especially in thebent portion of the U-shaped spring arm of the pressing member 22. Onthe other hand, it is desirable to form the pressing member from aplastic resin because it is cheaper than steel.

OBJECT OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a sheet film pack which prevents sheet film units from sufferingcreep strain even when they are stacked up for a long time on a platespring in a pack housing.

Another object of the present invention is to provide a creep resistantpressing member at a low cost, which is kept being compressed for a longtime in the sheet film pack.

SUMMARY OF THE INVENTION

In a sheet film pack which has a pack housing containing a plurality ofsheet film units in a stack therein, a lid slidable relative to the packhousing to open or close an open top of the pack housing, and a platespring mounted on an inner bottom surface of the pack housing to urgethe stack of sheet film units toward the open top, and in which eachsheet film unit consists of a sheet of photographic film and a filmsheath backing and fringing the sheet film, the invention makes theplate spring have a pair of arched arms which extend parallel to eachother along side zones of a bottom one of the sheet film units, suchthat peak areas of the arms contact the bottom sheet film unit atpositions within 15 mm from side edges of the bottom sheet film unit.

Because stiffness of the film sheath and thus the sheet film unit isrelatively large in the side zones, especially within 15 mm from theside edges thereof, deflection of the sheet film units supported on theplate spring is minimized, so that the creep-strain of the film sheathis prevented.

According to a preferred embodiment, the peak areas have a smaller widththan other portions of the arms, and the width of the peak areas ispreferably from 3 mm to 15 mm. It is preferable to provide the peakareas with protrusions protruding upward from upper surfaces of thearms.

If the sheet film pack has a counter disc held in a cavity of the packhousing so as to be rotated stepwise by an external counter advancingmechanism to indicate the number of sheet film units exposed in thesheet film pack, and a pressing member is mounted on the counter disc inthe cavity with a spring arm compressed in a vertical direction to applya pressure to the counter disc, wherein the spring arm extends from avertical side of a stem portion and has a curved portion and a freedistal end, the present invention makes the spring arm have differentcross-sectional areas in different portions thereof so that asubstantially equal stress may set up in the different portions of thespring arm under the compressed condition. According to a preferredembodiment, the curved portion has a larger cross-sectional area thanother portions of the spring arm.

Dispersing the stress over the spring arm is effective to prevent thecreep-strain of the spring arm that can occur especially in the curvedportion.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will becomeapparent in the following detailed description of the preferredembodiments when read in connection with the accompanying drawings,wherein like reference numerals designates like or corresponding partsthroughout the several views, and wherein:

FIG. 1 is a perspective view of a plate spring for use in a sheet filmpack according to an embodiment of the invention;

FIG. 2 is a top plan view of the plate spring shown in FIG. 1,illustrating the dimensions thereof;

FIG. 3 is a fragmentary sectional view illustrating an arched arm of theplate spring and a sheet film unit supported thereon, taken along a peakof the arm;

FIG. 4 is an explanatory side view of the plate spring;

FIG. 5 is a top plan view of the plate spring secured to a pack housingby caulking;

FIG. 6 is a vertical sectional view illustrating the caulked portion ofthe plate spring;

FIG. 7A is a top plan view illustrating a variation of protrusionsformed along the lengths of the arms;

FIGS. 7B and 7C are fragmentary top plan views illustrating furthervariations of protrusions formed along the lengths of the arms;

FIGS. 8A, 8B, 8C and 8D are sectional views illustrating possiblevariations in sectional contour of the protrusions;

FIG. 9 is a top plan view illustrating a plate spring having lugs assecuring members according to another embodiment of the invention;

FIG. 10 is a fragmentary sectional view illustrating the engagementbetween the lug of the spring plate and a hole of the pack housing;

FIG. 11 is a top plan view illustrating a plate spring having slits assecuring members of the plate spring which are force-fitted to buttonmembers formed on a pack housing;

FIG. 12 is a fragmentary sectional view illustrating the engagementbetween the slit and the button member;

FIG. 13 is an explanatory view illustrating a manufacturing method of aplate spring according to an embodiment of the invention;

FIG. 14 is an explanatory view illustrating a manufacturing method of aplate spring according to another embodiment of the invention;

FIG. 15 is a side view of a pressing member for pressing a counter discof the sheet film pack, according to a preferred embodiment of theinvention;

FIG. 16 is a front view of the pressing member viewed from a spring armthereof;

FIG. 17 is a table showing preferable physical properties of thepressing member;

FIG. 18 is a table showing stress values at different positions of thespring arm when compressed 7.7 mm in a vertical direction;

FIG. 19 is a table showing stress values at corresponding positions of acomparative spring arm having a constant cross-sectional area, whencompressed 7.7 mm in a vertical direction;

FIG. 20 is an exploded perspective view of a sheet film pack and a packholder for holding the sheet film pack according a prior art;

FIG. 21 is a sectional view of the sheet film pack;

FIG. 22 is a perspective view of a sheet film unit contained in thesheet film pack;

FIG. 23 is an exploded perspective view of the sheet film pack excludingthe sheet film unit;

FIG. 24 is a top plan view of a plate spring used in the prior art sheetfilm pack; and

FIG. 25 is a sectional view illustrating deflection of the sheet filmunits supported on the prior art plate spring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a plate spring 30 for use in a sheet film packaccording to an embodiment of the present invention. Because thefundamental configurations of the sheet film pack may be equivalent tothe sheet film pack shown in FIGS. 20 to 23, the following descriptionis limited to those parts essential to the present invention, while thesame or like parts are designated by the same reference numerals as inFIGS. 20 to 23.

The plate spring 30 has a pair of parallel arms 30a and 30b which areconnected at their one ends through a base portion 30c. The plate spring30 is blanked out as an integral body from a stainless steel springplate of 0.2 mm thick, e.g. "SUS 304 CSP". For example, an entire lengthD1 and a maximum width D2 of the arms 30a and 30b are 121 mm and 13.5mm, respectively. The base portion 30chas a length D3 of 104 mm and awidth D4 of 7.5 mm. The arms 30a and 30b have trapezoidal cutaways alonginside edges thereof so as to have a smaller width D7, e.g. 8 mm inintermediate portions 31 and 32 thereof, compared with the width D2.Shorter and longer sides of the trapezoidal cutaway have length D5 andD6 of 60 mm and 78 mm, respectively. The arms 30a and 30b are curvedalong the intermediate portions 31 and 32 to arch relative to an innerbottom surface 2a of a pack housing 2.

On the other hand, the stiffness of the film sheath 4 is higher in sidezones within 15 mm from side edges 4c and 4d thereof, than the remainingmiddle portion, if any pressure is applied to the side zones, and thefilm sheath 4 can be hard to deform. Since the arms 30a and 30b get intocontact with the sheet film unit 5 at peaks 31a and 32a of theirnarrower intermediate portions 31 and 32, the urging force of the platespring 30 is applied only to the side zones within 15 mm from the sideedges 4c and 4d of the film sheath 4 of the sheet film unit 5.Accordingly, the plate spring 30 is effective to prevent deflection andcreep strain of the film sheath 4. According to this embodiment, adistance D8 from the peaks 31a and 32a to an outer end 30f of the baseportion 30c is 69 mm.

In addition, a longitudinal ridge 35 or 36 is formed along an outer sideedge of either of the intermediate portions 31 and 32. The ridges 35 and36 has a length D9 of 36 mm and a width D10 of 1.5 mm, and is spaced bya distance D12 of 1.2 mm from the outer edge of each arm 30a or 30b. Asshown in FIG. 3, the ridges 35 and 36 have a height H1 of 0.5 mm. Forexample, the ridges 35 and 36 are formed by deep drawing to have atrapezoidal section, as shown in FIG. 3. Since the ridges 35 and 36 areformed along and proximate the outer side edges of the arms 30a and 30bin peak areas around the peaks 31a and 32a, the plate spring 30 pushesup the sheet film unit 5 at the stiffest portions of the sheet film unit5 near when the side edges 4c and 4d are formed. Thus, deflection of thesheet film units 5 are almost completely prevented.

As shown in FIG. 4, when no load is applied to the plate spring 30, aheight H2 from the bottom surface 2a of the pack housing 2 to the peak31a or 32a of each arm 30a and 30b, inclusive of the height of the ridge35 or 36, is 19 mm in this embodiment. The plate spring 30 further has asupporting member 38 formed to protrude inwardly from the base portion30c. The protrusion 38 has a length D14 of 60 mm and a width D15 of 7.5mm. The protrusion 38 is bent to incline toward the bottom surface 2a byan angle θ, e.g. 4°, relative to the base portion 30c. When the platespring 30 is depressed, the protrusion 38 is brought into contact withthe bottom surface 2a, and bears the load on the plate spring 30 incooperation with free ends 30d and 30e of the arms 30a and 30b, so thatthe outer end 30f of the base portion 30c is prevented from removing offthe bottom surface 2a that may otherwise be caused by acompression-deflection of the plate spring 30.

A hole 41 or 42 is formed through the end portion of each arm 30a or 30bthat is closer to the base portion 30c. According to this embodiment,the holes 41 and 42 are spaced a distance D16 of 84 mm from each otherand a distance D17 of 15 mm from the outer end 30f of the base portion30c. As shown in FIGS. 5 and 6, the holes 41 and 42 are fitted on a pairof bosses 44 which are formed on the bottom surface 2a of the packhousing 2. Thereafter, the bosses 44 are melted to secure the platespring 30 to the bottom surface 2a by caulking. In this way, the platespring 30 is positioned exactly on the bottom surface 2a without anyplay. After the caulking, a height H3 of a top surface 44a of the boss44f relative to the bottom surface 2a of the pack housing 2 should be solow as not to hinder the sheet film unit 5 that is moving back to thepack housing 2 through a recovery mouth 8 after the exposure.

Spring force of the arms 30a and 30b of the plate spring 30 is 100 g to600 g, preferably 200 g to 400 g. Where the spring force is less than100 g, the topmost sheet film unit 5a cannot reliably pop out of thepack housing 2. Moreover, tightness between the sliding lid 9 and thepack housing 2, namely between the sliding lid 9 and the guide rails 2cin the closed position is insufficient so that the sliding lid 9 cannotreliably shield the interior of the pack housing 2 from ambient light.Where the plate spring 30 has a spring force of more than 600 g, thesheet film units 5 are apt to deflect, and friction between the undersurface of the sliding lid 9 and the topmost sheet film unit 5a is toolarge for smooth reciprocating operation of the sliding lid 9.

Test data for proving the advantages of the present invention will beset forth below.

Table 1 shows the degree of deflection of the film sheath 4 detectedwhile changing contacting positions 30a and 30b of the plate spring 30with the film sheath 4, concretely the distance of each contactingposition from the side edge 4c or 4d of the film sheath 4. In Table 1,characters "A", "B", "C", "D" and "E" represent the degrees ofprevention against deflection "A" represents very good, "B" good, "IC"tolerable for practical use, "D" necessary to improve, and "E" tolerablefor practical use.

                  TABLE 1                                                         ______________________________________                                        CONTACT POSITION DEFLECTION                                                   ______________________________________                                         1 mm            A                                                             2 mm            A                                                             3 mm            A                                                             5 mm            B                                                            10 mm            B                                                            15 mm            C                                                            20 mm            D                                                            25 mm            E                                                            ______________________________________                                    

The results in Table 1 show that the contacting positions of the arms30a and 30b with the film sheath 4 should be disposed within 15 mm fromthe side edges 4c and 4d of the film sheath 4, respectively, so as toprevent such deflection and the creep strain of the film sheath 4 asshown in FIG. 25. Since the maximum width D2 of the arms 30a and 30b is13.5 mm in the above embodiment, the plate spring 30 would remarkablyreduce the deflection of the film sheath 4 without the narrowerintermediate portions 31 and 32 or the ridges 35 and 36. However, thenarrower intermediate portions 31 and 32 ensure the arms 30a and 30bcontacting the film sheath 4 at positions within 15 mm from the sideedges 4c and 4d.

Table 2 shows the results of experiments on the degree of deflection ofthe film sheath 4 and formability of the plate spring 30 in relation tothe width D7 of the intermediate portions 31 and 32, i.e. of the peaks31a and 32a of the arms 30a and 30b.

                  TABLE 2                                                         ______________________________________                                        WIDTH D7     DEFLECTION FORMABILITY                                           ______________________________________                                         1 mm        A          E                                                      2 mm        A          D                                                      3 mm        A          C                                                      5 mm        B          B                                                     10 mm        B          B                                                     15 mm        C          B                                                     20 mm        D          A                                                     25 mm        E          A                                                     ______________________________________                                    

As shown in Table b 2, the width D7 of the arms 30a and 30b at the peaks31a and 32a should not be more than 15 mm in view of the deflection ofthe film sheath 4, but should not be less than 3 mm in view offormability of the plate spring 30. Therefore, the width D7 ispreferably 3 mm to 15 mm. Since the width D7 is 8 mm in the abovedescribed embodiment, the plate spring 30 can be easy to mold, but canhardly cause the deflection or creep strain in the film sheath 4.

As described above, the ridges 35 and 36 are formed by deep drawing ametal spring plate. But the facility of deep drawing varies dependingupon the width D10 of the ridges 35 and 36. Table 3 shows the results ofexperiments on the facility of deep drawing in relation to the widthD10.

                  TABLE 3                                                         ______________________________________                                                     DEEP DRAWING                                                     WIDTH D10    FACILITY                                                         ______________________________________                                        0.3 mm       E                                                                0.1 mm       D                                                                1.0 mm       C                                                                1.5 mm       B                                                                2.0 mm       B                                                                3.0 mm       A                                                                5.0 mm       A                                                                ______________________________________                                    

Table 3 shows that the width D10 of the ridges 35 and 36 should be 1.0mm or more. Since the width D10 is 1.5 mm in the above describedembodiment, the ridges 35 and 36 are suitable for deep draw forming.

The length D9 of the ridges 35 and 36 may be equal to the entire lengthD1 of the arms 30a and 30b, but it was found preferable to provide theridges 35 and 36 within a range of ±20 mm around the peaks 31a and 32a,for preventing deflection of the film sheath 4, while ensuring goodmoldability of the ridges 35 and 36 and suitable resilience of the platespring 30. Therefore, the length D9 of the ridges 35 and 36 isdetermined to be 36 mm in the present embodiment. It was also provedthat a preferable range of the height H1 of the ridges 35 and 36 is from0.1 mm to 1.0 mm, in view of prevention against the deflection of thefilm sheath 4, moldability of the ridges 35 and 36, and resilience ofthe plate spring 30. Therefore, the height H1 of the ridges 35 and 36 isdetermined to be 0.5 mm in the present embodiment.

The present invention should not be limited to the above-describedembodiment. For example, each of the ridges 35 and 36 may be replaced bya plurality of, e.g., three shorter ridges 51a, 51b and 51c aligned in aline, as shown in FIG. 7A, or by a plurality of, e.g., eight bossesaligned in a line, as shown in FIG. 7B. It is also possible to combinethese shorter ridges 51a and 51b and bosses 52a and 52b, as shown inFIG. 7C.

Also the ridges 35 and 36 may have another sectional contour than thetrapezoidal section shown in FIG. 3. For example, as shown in FIGS. 8Ato 8D, it is possible to form a ridge 35a, 35b, 35c or 35d having anarched, triangular, rectangular, or polygonal sectional contour,respectively. It is also possible to cement separate parts onto theplate spring to provide ridges.

Although the plate spring 30 is secured to the bottom surface 2a of thepack housing 2 by caulking in the above embodiment, the plate spring 30may be secured to the bottom surface 2a in the same way as the prior artshown in FIG. 21, that is, by contacting the outer end 30f of the baseportion 30c against a lateral ridge 54 formed on the bottom surface 2a.It is also possible to form lugs 56 integrally with a plate spring 55,which protrude laterally outwardly from opposite sides of the baseportion 30c, as shown in FIG. 9. In that case, recesses 57 are formed ininner side walls of a pack housing 2 so as to accept the lugs 56, asshown in FIG. 10. Alternatively, the plate spring may have recesses inthe lateral sides, whereas corresponding projections are formed on theinner side walls of the pack housing.

According to another embodiment shown in FIGS. 11 and 12, a pair ofbutton members 58 are formed integrally on a bottom surface 2a of a packhousing 2. The protrusion 58 has a flat top 58a and a stem 58b with asmaller diameter. Corresponding to the protrusions 58, a pair ofcut-outs 59 are formed in a plate spring 60, so that the cut-outs 59 areforce-fitted onto the stems 58b of the button members 58 by sliding theplate spring 60 in a direction shown by arrows in FIG. 11.

Although the plate spring 30 is formed by blanking out a metal springplate into the complete shape in the above embodiments, it is possibleto form a plate spring 62 by folding a linear metal strip 63, which isblanked out from a metal spring plate, into the predetermined shape asshown by phantom lines in FIG. 13.

Furthermore, it is possible to form a plate spring 65 by connectingseparate arm members 65a and 65b through a base member 65c, as is shownin FIG. 14. In this embodiment, the arm members 65a and 65b arepreferably made of metal spring plates, while the base member 65c may bea metal plate or a plastic plate. For joining these members 65a to 65c,welding or bonding is applicable. When the base member 65c is a plasticplate, it is preferable to provide the base member 65c with grooves orthe like for accepting ends of the arm members 65a and 65b, so as toensure the joints.

It is to be noted that the above mentioned stainless steel "SUS 340 CSP"is suitable for the material of the plate spring, because it hardlyrusts without any surface treatment, does not adversely affect thephotosensitivity of the sheet film 3, and is inexpensive. However, anyother conventional elastic materials suitable for a spring, such asberyllium steel, phosphor bronze, engineering plastics and so forth, areapplicable. Of course, the plate spring may be taken surface treatmentsuch as rust preventing treatment, antistatic treatment, metal-deposittreatment, and so forth.

Referring now to FIG. 15 showing a pressing member 100 for use with acounter disc 21, the pressing member 100 according to the invention isimproved in creep-resistance, as will be described below. The pressingmember 100 fundamentally consists of a stem portion 101 and a spring arm102 like the conventional pressing member 22. The stem portion 101 has abox-shaped stem base 101a and a guide portion 101c. A top surface 101aaof the stem base 101a is higher than an upper end 101ca of the guideportion 101c. Grooves 101cb are formed on opposite surfaces of the guideportion 101c, which are engaged in a vertical rail 20a of a cavity 20 ofthe pack housing 2, to guide the pressing member 100 down into thecavity 20.

The spring arm 102 extends from a middle portion of a vertical side101ac of the stem base 101a opposite from the guide portion 101c,substantially perpendicularly to the vertical side 101ac. Hereinafter, aportion 102a of the spring arm 102 that extends substantially horizontalwill be referred to as a horizontal arm portion 102a. The horizontal armportion 102a is connected to a fore arm portion 102c through a curvedportion 102b. According to the present invention, the fore arm portion102c does not extend horizontally, but obliquely to the horizontal armportion 102a. A box-shaped distal end portion 102d is formed in thedistal end of the spring arm 102, such that an outer end surface 102d aof the distal end portion 102d is put in contact with the inner surfaceof the sliding lid 9 to constrain or compress the pressing member 100 inthe cavity 20.

In the constrained position, a bottom surface 101ab of the stem portion101 is pressed against one of a plurality of anti-reversal claws 21a ofthe counter disc 21. If the pressing power of the pressing member 100 istoo small, the counter disc 21 cannot stop properly, then it would bedifficult to read indicia on the counter disc 21. If, on the other hand,the pressing power is too large, it would be difficult to smoothlyadvance the counter disc 21 by an external counter advancing mechanism.For applying an appropriate pressing power onto the counter disc 21, thepressing member 100 should preferably have a reactive force of 20 g to100 g, and most preferably 50 g, when the pressing member 100 iscompressed 7.7 mm in the vertical direction. It is to be noted that thepressing member 100 is assumed to be compressed vertically by 7.7 mm bythe sliding lid 9 when a predetermined number of sheet film units 5 arecontained in the pack housing 2.

The present invention determines the angle of the fore arm portion 102crelative to the horizontal direction so that the pressing member 100 mayhave a reactive force in that range when compressed vertically by 7.7mm. Preferably, the inner surface of the fore arm portion 102c forms anangle θ1 of 32° to 72° relative to the horizontal direction, and theouter surface of the fore arm portion 102c forms an angle θ2 of 30° to70° relative to the horizontal direction. The most preferable angles θ1and θ2 are 52° and 50°, respectively.

The distal end portion 102d is not a rectangular prism, but the outerend surface 102da and an adjacent end surface 102db form an angle θ3 of83°, so that the outer end surface 102d a may tightly contact the innersurface of the sliding lid 9, while the pressing member 100 isconstrained by the sliding lid 9 and thus compressed vertically 7.7 mm.The angle θ3 may preferably be in a range from 73° to 93°, but mostpreferably 83°.

According to a preferred embodiment, a root 102ac of the spring arm 102where the spring arm 102 is connected to the stem portion 101, has athickness T1 of 0.9 mm. The thickness decreases from the root 102actoward a middle position 102aa of the horizontal arm portion 102a, sothat the middle position 102a a has a thickness T2 of 0.6 mm. From themiddle position 102aa, the thickness increases toward the curved portion102b. As a result, a thickness T3 of a connecting position 102ab betweenthe horizontal arm portion 102a and the curved portion 102b is 1.0 mm.It is to be noted that "thickness" here means a distance from a point ofthe outer surface of the spring arm 102 to a second point of the innersurface that is disposed in the same normal line to the outer surface asthe first point.

Of the horizontal arm portion 102a, a length L1 from the vertical side101ac of the stem portion 101 to the connecting position 102ab is 14.5mm, whereas a length L2 from the vertical side 101ac of the stem portion101 to the middle position 102aa is 7.75 mm. Upper and lower bordersbetween the root 102ac of the spring arm 102 and the stem portion 101 isrounded with radii R1 and R2 of curvature of 1.5 mm and 1 mm,respectively.

The curved portion 102b has a thickness T4 of 1 mm throughout thelength, i.e., from the connecting position 102a b to another connectingposition 102cb connected to the fore arm portion 102c. The inner andouter surfaces of the spring arm 102 in the curved portion 102bhaveradii of curvature of 2.2 mm and 3.2 mm, respectively, relative to acommon center point which is disposed 14.5 mm distant from the verticalside 101ac in the horizontal direction, and 8.8 mm from the bottomsurface 101ab of the stem portion 101 in the vertical direction.

The thickness of the fore arm portion 102c decreases from the connectingposition 102cb of 1.0 mm thick toward a distal connecting position 102cahaving a thickness T5 of 0.6 mm, where the fore arm portion 102c isconnected to the distal end portion 102d. The angle θ1 of the innersurface of the fore arm portion 102c is 52° relative to the horizontaldirection, whereas the angle θ2 of the outer surface of the fore armportion 102c is 50° relative to the horizontal direction.

The angle θ3 of the outer end surface 102da to the adjacent end surface102db is 83°. A length L3 from the end surface 102db to the oppositeside 102ddof the distal end portion 102d is 2.8 mm. The end surface102db has a length L4 of 3.8 mm in a direction from the outer endsurface 102da to an inner end surface 102dc of the distal end portion102d, while a length or distance L5 from the outer end surface 102da tothe inner end surface 102dc is 3.6 mm in the opposite side 102dd to theend surface 102db. A vertical length or height L6 from the bottomsurface 101ab of the stem portion 101 to an upper tip of the distal endportion 102d is 24.1 mm.

FIG. 16 schematically shows the pressing member 100 viewed from thefront of the spring arm 102. As shown, the spring arm 102 has a constantwidth W1 of 2 mm throughout the entire length.

The pressing member 100 is formed as an integral body by resin injectionmolding. As for the resin, any material is applicable if only it issuperior in lightweight properties and workability, and is able toprevent creep strain of the spring arm 102 for a sufficiently long time.Preferred are polyurethane resin, polyester resin, acrylic resin,polyacetal resin, polyphenylene oxide resin, etc., of which polyacetalresin is the most preferable. It is preferable to form the pressingmember 100 from polyacetal resin by use of an in-line screw typeinjection molding device, at a clamp pressure of 75t, in a molding cycleof 18 seconds, with a resin temperature of 215° and a mold temperatureof 80°. However, these conditions are not limitative.

Polyacetal resin generally contains formaldehyde. It is well known thathigh density formaldehyde has a bad effect on photosensitivity. That is,it would deteriorate the photosensitive emulsion layer of the sheetfilm. Therefore, in case of the pressing member 100 being formed frompolyacetal resin, the density of formaldehyde emerging from the pressingmember 100 must be 30 ppm or less, and more preferably 20 ppm or less inthe interior of the pack housing 2.

The pressing member 100 formed from polyacetal resin preferably hasphysical properties as shown in FIG. 17, through they are variable sofar as an appropriate reactive force and a sufficient creep-resistancecan be obtained in the pressing member 100.

The pressing arm 102 of the pressing member 100 configured as above andformed from polyacetal resin has a reactive force of 50 g when thedistal end portion 102d is depressed to compress the vertical length L6by 7.7 mm. FIG. 18 shows the values of stress at different portions orpositions of the spring arm 102 when it is depressed or compressed inthat way. The stress values are calculated according to the finiteelement method after modeling the configurations of the pressing member100 by use of a general purpose analyser program software.

As shown in FIG. 18, the stress at a middle position 102ba of the curvedportion 102b is 147 kgf/cm² (14.4 MPa), that is the largest of allportions of the spring arm 102. However, the smallest stress at themiddle position 102aa of the horizontal arm portion 102a is 0.7 kgf/cm²(0.07 MPa), and the difference between the largest and the smalleststress values is 146.3 kgf/cm² (14.4 MPa).

As comparative data, FIG. 19 shows stress values at the correspondingpositions of a spring arm of a pressing member which has almost the sameconfigurations as the pressing member 100, except that the thickness aswell as the width of its spring arm is constant. The comparativepressing member was formed from polyacetal resin so as to have areactive force of 50 g when its distal end portion 102d is depressed tocompress the comparative pressing member vertically by 7.7 mm. Thestress values are calculated according to the finite element methodafter modeling the configurations of the comparative pressing member byuse of the general purpose analyser program software.

As shown in FIG. 19, in the comparative pressing arm, the largest stressat the middle position 102ba of the curved portion 102b is 185 kgf/cm²(18.1 MPa), whereas the smallest stress at the middle position 102aa ofthe horizontal arm portion 102a is 4 kgf/cm² (0.39 MPa). Accordingly,the difference between the largest and the smallest stress values is 181kgf/cm² (17.8 MPa).

Consequently, in the spring arm 102 of the present invention, themaximum difference in stress between the respective portions is small,and also the largest stress in the curved portion 102b is remarkably,i.e., about 20%, reduced, compared with the comparative pressing arm.

In conclusion, as a result of making the root 102a c and the curvedportion 102b of the spring arm 102 thicker than the middle position 102aa of the horizontal arm portion 102a and the distal connecting position102ca of the fore arm portion 102c, the stress under compression of thespring arm 102 is dispersed toward the distal connecting position 102caof the fore arm portion 102c, thereby reducing the stress concentrationon the curved portion 102b. Thus, creep-strain of the spring arm 102 isminimized or prevented.

To prevent the stress concentration on the curved portion 102b, it ispossible to change the width of the spring arm 102 instead of thethickness thereof. In other words, stress distribution depends upon thecross-sectional areas of the respective portions of the spring arm. Forexample, by making the root 102a c and the curved portion 102b of thespring arm 102 wider than the middle position 102a a of the horizontalarm portion 102a and the distal connecting position 102ca of the forearm portion 102c, the compression stress is dispersed toward the distalconnecting position 102ca of the fore arm portion 102c, so that the sameeffect as above is achieved.

Although the present invention has been described in detail with respectto the preferred embodiments shown in the drawings, the presentinvention is not to be limited to those embodiments.

For example, the pressing member 100 may be constrained by a cover plateput on the open top of the cavity 20, instead of the sliding lid.

The plate spring or the pressing member according to the presentinvention is not only useful in other type sheet film pack, but alsoapplicable to other devices that use an elastic member.

Thus, various modifications may be possible to those skilled in the artwithout departing from the scope of the appended claims.

What is claimed is:
 1. A sheet film pack comprising:a pack housing containing a plurality of sheet film units in a stack therein; a counter disc held in a cavity of said pack housing so as to be rotated stepwise by an external counter advancing mechanism to indicate the number of sheet film units exposed in said sheet film pack; and a pressing member having a stem portion and a spring arm extending from a vertical side of said stem portion and having a curved portion and a free distal end, said pressing member being mounted on said counter disc in said cavity with said spring arm compressed in a vertical direction to apply a pressure to said counter disc, said spring arm having different cross-sectional areas in different portions thereof so that a substantially equal stress may be set up in the different portions of said spring arm under the compressed condition, and wherein said curved portion of said spring arm has a larger cross-sectional area than other portions of said spring arm.
 2. A sheet film pack as recited in claim 1, wherein the cross-sectional area of said spring arm is the largest at a root of said spring arm connected to said stem portion as well as in said curved portion, and the smallest at a middle position between said root and said curved portion, and the cross-sectional area decreases from said curved portion toward said distal end of said spring arm.
 3. A sheet film pack as recited in claim 2, wherein the cross-sectional area of said spring arm is changed by changing the thickness of said spring arm.
 4. A sheet film pack as recited in claim 2, wherein the cross-sectional area of said spring arm is changed by changing the width of said spring arm.
 5. A sheet film pack as recited in claim 1, wherein a box-shaped portion is formed on said distal end of said spring arm such that said spring arm is compressed by being depressed at an end surface of said box-shaped portion.
 6. A sheet film pack comprising:a pack housing containing a plurality of sheet film units in a stack therein; a counter disc held in a cavity of said pack housing so as to be rotated stepwise by an external counter advancing mechanism to indicate the number of sheet film units exposed in said sheet film pack; and a pressing member having a stem portion and a spring arm extending from a vertical side of said stem portion and having a curved portion and a free distal end, said pressing member being mounted on said counter disc in said cavity with said spring arm compressed in a vertical direction to apply a pressure to said counter disc, said spring arm having different cross-sectional areas in different portions thereof so that a substantially equal stress may be set up in the different portions of said spring arm under the compressed condition, wherein a first portion of said spring arm that extends from said root to said curved portion is substantially perpendicular to said vertical side of said stem portion, and a second portion of said spring arm that extends from said curved portion to said distal end is oblique to said first portion.
 7. A sheet film pack as recited in claim 6, wherein the angle of the second portion relative to the first portion is 32° to 72° in the inner surface of the second portion, and 30° to 70° in the outer surface of the second portion, such that said pressing member has a reactive force of 20 g to 100 g while compressed vertically 7.7 mm.
 8. A sheet film pack comprising:a pack housing containing a plurality of sheet film units in a stack therein; a counter disc held in a cavity of said pack housing so as to be rotated stepwise by an external counter advancing mechanism to indicate the number of sheet film units exposed in said sheet film pack; and a pressing member having a stem portion and a spring arm extending from a vertical side of said stem portion and having a curved portion and a free distal end, said pressing member being mounted on said counter disc in said cavity with said spring arm compressed in a vertical direction to apply a pressure to said counter disc, said spring arm having different cross-sectional areas in different portions thereof so that a substantially equal stress may be set up in the different portions of said spring arm under the compressed condition, wherein said stem portion and said spring arm are formed as one body from polyacental resin. 